Optical communications systems have been developed in recent years in response to the demand for high capacity and long distance communications systems. Such optical communications systems as presently contemplated have a light source and a photodetector that are optically coupled to each other by means of a glass transmission line. The glass transmission line is commonly referred to as an "optical fiber" by those skilled in the art and the optical fibers presently contemplated for communications use are silica-based compositions.
For highest capacity operation and for longest distance operation without need for repeaters, a single frequency semiconductor laser is presently the light source of choice. The term "single frequency" as used in this application means a narrow spectral output corresponding to single longitudinal mode operation. A laser with such an output characteristic minimizes problems, e.g., limited repeater spacings or lower data rates, associated with optical fiber dispersion. The wavelength range between 1.3 and 1.6 .mu.m is presently the wavelength range of greatest interest because within this range both dispersion and optical loss of the optical fiber are at a minimum although not necessarily at the same wavelength.
It should be noted that it has become apparent to those skilled in the art that the term "single longitudinal mode" is capable of several interpretations from the viewpoint of optical communications systems. The observation of single longitudinal mode output from a semiconductor laser under steady state excitation does not guarantee that the laser will emit a single frequency output under actual modulation conditions. This statement is especially true at very high bit rates and when the fibers are not dispersion free at the operating wavelength. Accordingly, to those skilled in the optical communications art at the present time, the term "single longitudinal mode output" has come to mean single longitudinal mode output from a, for example, semiconductor injection, laser when it is being modulated under actual, for example, high data rate, operating conditions.
A variety of approaches has been taken in attempts to develop single frequency, that is, single longitudinal mode operation, semiconductor lasers. For example, single frequency operation in semiconductor lasers has been achieved by several approaches, including distributed feedback (DFB), distributed Bragg reflector (DBR), injection locking, coupled cavity lasers, and very short cavity lasers. See, for example, Electronics Letters, 18, pp. 77-78, Jan. 21, 1982; Electronics Letters, 18, pp. 410-411, May 13, 1982; Electronics Letters, 18, pp. 445-447, May 27, 1982; Applied Physics Letters, 38, pp. 315-317, Mar. 1, 1981; and IEEE Journal of Quantum Electronics, QE-18, pp. 1101-1113, July 1982.
While these approaches may be perfectly adequate for many applications, many of them suffer drawbacks such as complicated fabrication techniques. This drawback is especially true for the DFB and DBR lasers. It has also been observed for several of these approaches that stable single longitudinal mode operation occurs over only relatively narrow current injection ranges.